I’ve had this happen a couple times over the past 2 months. Not exactly sure what the cause was, but I’m fairly certain both players didn’t drop internet connection. This seems to affect GW coop auto-matchups; I haven’t encountered this on coop with friends.

If the other player leaves during/after the mission, then the play again button will not be available since there is nobody to send the invite to.

Edited to add: I don’t ever run my gorgon co-ops unless I have to. If I’m only at 2/3 coops for the daily quest (2 from a dojo/CH trade) then I’ll play one gorgon on auto and just close the app because the gorgon co-op is agonizingly boring. That way I still get the daily quest for 3 co-op wins.